Whisk together 1/3 cup ketchup, 1/3 cup apple cider vinegar, 2 Tbsp. sugar, 1/2 tsp. kosher salt, 1/2 tsp. freshly ground black pepper, and 1/4 tsp. hot sauce in a bowl. Add 1/2 head cabbage (about 1 lb.), grated, and 1/4 large sweet onion, grated; toss to coat. Let stand 1 hour before serving, tossing occasionally.